1. Field of the Invention
The present invention relates to the firing of thyristors by a forward overvoltage.
2. Description of the Prior Art
A thyristor is a solid state four layer PNPN semiconductor device which supports a high voltage without significant conduction of current when in its high impedance blocking or "off" state; and conducts current when in its low impedance conducting or "on" state. Thyristors are capable of being turned on or fired by causing a threshold emitter current to flow. This cathode emitter current is primarily made up of electrons that are injected into the cathode or P-base region. These injected electrons induce anode-to-cathode current to flow and the thyristor fires by regenerative action. Thyristors may be either of the two terminal or three terminal types. In the three terminal type of thyristor, the cathode emitter current that causes the thyristor to fire is generated by applying a forward voltage between its cathode electrode connected to its cathode emitter layer or region and its gate electrode connected to its cathode base region. In two terminal thyristors, this cathode emitter current flows through the anode emitter. Thus, the difference between the two terminal and three terminal types of thyristor is primarily a matter of the manner in which the cathode emitter current which causes the thyristor to fire, is generated. In the case of the two terminal type, this cathode emitter current is generated by either of two means. The current can be created by increasing the anode-to-cathode voltage to the point at which the blocking PN junction (i.e., the junction between the cathode base and the anode base) loses its blocking ability (overvoltage) and starts to permit current flow. This usually occurs at the avalanche voltage of the blocking PN junction. Or, the emitter cathode current that fires the device can be generated by means of a rapidly increasing anode to cathode voltage that causes a displacement current to flow as the depletion layer capacitance of the junction between the cathode base and anode base changes its charge.
Commercially available two terminal thyristors are somewhat deficient in that they turn on at some randomly located non-predetermined point, the location of which depends on uncontrolled nonuniformities in the device. Such nonuniformities can make the performance of the device nonreproducible and cause it to be subject to unnecessary switching losses; and also subject to failure. The unnecessary switching losses are related to the fact that the area of the initial turn-on of the device is uncontrolled and larger than necessary. Thus, the build-up of current and charge density is slower than necessary. It is understood that switching losses are decreased by increasing the speed of turn-on of any thyristor.
In three terminal thyristors, it is known that the switching speed and losses are improved by building a small pilot thyristor within the area of the thyristor device. A given gate drive current, in this case, fires the small area pilot thyristor rapidly; and this, in turn, provides from the anode circuit, a large drive current that, in turn, fires the main thyristor rapidly and efficiently. However, even three terminal devices may be fired, intentionally or unintentionally, by the application of an overvoltage between its anode and cathode electrodes in the same manner as the two terminal device.
Thus, it has been proposed that a pilot thyristor may be utilized in a two terminal device to provide for more rapid firing or turn-on. In addition, various structures have been proposed to insure that the pilot thyristor fires prior to the firing of the main thyristor in response to an overvoltage.
For example, in U.S. Pat. No. 3,766,450 there is shown a three terminal thyristor with an auxiliary thyristor portion centrally located in the device where the portion of the anode base underlying the central portion of the cathode base to which the gate electrode is connected is formed with a higher impurity concentration than that of the anode base portion lying outwardly of such area. In this manner, the U.S. Pat. No. 3,766,450 teaches that the application of a voltage applied across the main anode to cathode terminals of the thyristor which exceeds the forward breakdown voltage insures that the auxiliary emitter is triggered ahead of the main emitter of the device. Also, U.S. Pat. No. 3,774,085 shows a three terminal thyristor, without an auxiliary thyristor portion, the anode or N-base which has an area of higher impurity concentration below the cathode and gate electrodes than that concentration of the N-base situated outwardly of the selected area in order that the thyristor is gated or triggered within an area encompassed by the electrodes when a breakover voltage is applied to the anode electrode. Although such proposed devices provided advantages in the operation of a thyristor in response to an overvoltage, it is realized that they are difficult to manufacture, in that the region of higher impurity concentration had to be present in the starting slice. This is difficult to produce. In the prior art the higher impurity concentration region is the result of natural area nonuniformities that occur when the ingot is grown. These are difficult to control.
In U.S. Pat. No. 3,906,545, there is shown a three terminal thyristor device that utilizes a higher impurity region within the cathode base of the device of the same conductivity type. In this prior art device, the higher doped P base region extends in the form of a plurality of narrow paths which emanate from the gate electrode and spread over the entire cross-sectional area of the gate zone. The apparent objective of that invention is to decrease lateral resistance in the cathode base, and would not effect the breakdown voltage of the junction between the cathode and anode bases.
Thus, it is desirable to provide a thyristor that may be either of the two terminal or three terminal type; and may be fired either intentionally or unintentionally, by overvoltage, with low switching losses, and in a predictable manner, and without danger of degrading or failure of the device. At the same time, it is desirable that such device be practical to manufacture and simple in its design.
In accordance with the present invention, and in contrast to the known prior art, a two terminal thyristor device has a centrally located auxiliary emitter portion with an extra P type diffusion in the cathode base region, located within the outer boundaries of such auxiliary emitter portion which locally decreases the avalanche voltage somewhat; and serves the purpose of insuring that in response to a high forward voltage, the avalanche will occur in the area of the extra diffusion. Further, in keeping with the invention, either a two or a three terminal device may include an additional P type diffusion layer in the cathode base in the central portion of the device and slightly overlap the inner boundaries of the auxiliary emitter. This extra diffusion layer has a higher dopant density gradient, adjacent its junction with the anode base, than the regular P type diffusion layer that is outside of the extra diffusion region. This also insures that the avalanche will occur along the inner edge of the auxiliary emitter in an area where its electric field is highest in depletion layer at the junction between its cathode and anode bases. This provides for a further advantage in that additional carriers can be generated by carrier multiplication.
In contrast to the prior art, by making the diffused forward blocking PN junction more abrupt, rather than changing the dopant density in the anode base, the reduction in breakdown voltage is more easily kept small and reproducible. In principle, one should decrease the breakdown voltage only enough to bring it below the lowest breakdown voltage that would occur without the benefit of the present invention. A decrease in breakdown voltage beyond that necessary to insure that the designated region has the lowest breakdown voltage, needlessly degrades the performance of the thyristor. Because most of the breakdown voltage is supported in the depletion layer in the anode base region, changes in the cathode base produce only small, albeit adequate, decreases in the breakdown voltage. Hence, large process variations produce small changes in the breakdown voltage, and this increases the manufacturability of the device.